Simple linear amines are straight chain compounds that contain nitrogen in the molecular chain. Examples include ethylenediamine, diethylenetriamine, triethylenetetramine, and tetraethylenepentamine. These amines are commercially available as mixtures of branched and linear molecules of a predetermined molecular weight range. For example, tetraethylenepentamine is generally available in the molecular weight range centered on 189.
Simple linear amines are known to form complexes with cations. Several of the above-mentioned linear amines have high stability coefficients for specific cations. For example, the stability coefficients for tetraethylenepentamine with cations of nickel, copper, lead, and zinc are reported to be 10.sup.17.6, 10.sup.23.1, and 10.sup.15.3, respectively. These linear amines are liquids at ambient temperatures, and they are soluble in water solutions. They have been immobilized by chemically binding them to silicate surfaces to produce insoluble particles that will form complexes with and retain such selected cations. Thus, that simple amines will form highly stable complexes with cations is well known. If a simple amine were immobilized by chemical bond attachment, it would be expected to retain the ability to form a metal-cation complex at least to a limited degree.
Techniques for binding an amine to silicon dioxide or silicon hydroxide, including glass, to aluminas, and to other insoluble elemental oxides are known. For example, the amine may be reacted with chloropropyltrimethoxysilane, chloropropyltriethoxysilane, or any combination of chloromethyl, or -ethyl, or -propyl, or -butyl etc. with any combination of mono- or di- or tri- methoxy or ethoxy or propoxy, or hydroxy, etc, plus binding of the silane to the silicon dioxide, as a method of immobilizing ligands as a solid phase. Specifically, the immobilization of tetraethylenepentamine by this technique is known from Czech Pat. No. 177,563 to Popus Vynalezu. It is also known from U.S Pat. No. 4,203,952 to Hancock to bind a polyamine such as triethylenetetramine to silica by use of the coupling agent alphachloropropyl trimethoxy silane and to use this resulting bound ligand as a metal chelating agent. The same method has been used to immobilize monamine and ethylene diamine, and these products have been used as metal chelating agents. U.S. Pat. No. 4,448,694 to Plueddemann employs this method to bind both ends of the amine to the silica.
Polyethyleneimine (PEI) is composed of amines of the type H.sub.2 N[(CH.sub.2).sub.m NH].sub.n H, where m=2 to 5 and n=8 to 1200. These PEIs have been prepared with molecular weights as high as 60,000. PEIs are produced in large quantities in molecular weight ranges of 1,200 and of 50,000. These larger molecules, in molecular weight ranges from about 400 to 60,000 probably are branched and cross linked, rather than being in absolute linear form. All the molecular weight ranges are referred to as polyethyleneimine of specified average molecular weight, such as PEI-1200.
PEI is extremely viscous. Ions diffuse through and into PEI very slowly. Generally, for this reason, PEI has not been useful as an organic solvent. Similarly, PEI generally has not been a likely choice to be adsorbed in the pores of physical adsorbents such as charcoal or clay, since it could be foreseen that a low diffusion rate of ions into PEI would inhibit the ion complexation to the degree that a useful application of the material would be difficult.
Polyethyleneimine has been known to be linked to silica surfaces, which has made polyethyleneimine useful as a chromatographic column packing and in the purification and separation of anions. For example, U.S. Pat. No. 4,540,486 to Ramsden discloses the preparation of a product that is polyethyleneimine bound by silane to silica gel. This product is formed of silica gel having an average particle diameter of from about three to seventy microns. Apparently, this relatively small particle diameter is desirable in a chromatographic column, although such small particles might be expected to cause a high pressure drop. It could be expected that the smallness of the particles would permit the loading of particles into extremely small columns, and ions rapidly could diffuse into small particles. Thus, such small particles might be desirable in an analysis-process chromatographic column, which requires rapid diffusion.
The process of making the Ramsden product employs the reaction of polyethyleniminopropyl trimethoxy silane with silica gel. The reason for reacting the PEI with silane before bonding to the silica gel is not stated, although it may be to avoid cross-linking in the product. The absence of cross-linking apparently is important to the usefulness of product, according to the Ramsden disclosure.
The Ramsden patent includes further teaching that ion selectivity is known to the extent that carboxylated polyethyleniminopropyl-silyl-silica gel will complex the cations of proteins. Further, polyethylenimino-propylsilyl-silica gel will complex anionic protein solution.
A related disclosure is found in European Patent No. 403,700 to Baker, Inc., in which a covalently bound, non-crosslinked PEI silica based solid phase support serves as an affinity chromatography matrix that is stable in various environments and is selective in adsorption of specific molecules.
The preparation of surface-modified silicas for use as ion-exchangers is disclosed in Janzen, Unger, Muller and Hearn, Adsorption of Proteins on Porous and Non-porous Poly(ethyleneimine) and Tentacle-Type Anion Exchangers, Journal of Chromatography, 522 (1990) 77-93.
A known application of immobilized simple linear amines is the complexation retention of ions, such as the generally unwanted ions of bismuth, cobalt, chromium, copper, gold, iron, lead, mercury, nickel, radium, silver, tin, and zinc at small concentrations of those ions, even in parts per billion, while not appreciably complexing the harmless ions of sodium, potassium, calcium and magnesium at concentrations even thousands of times greater. Because the immobilized-ligand capacity is not taken up by the harmless ions, the loading-cycle time of the ligand before it is fully loaded can be longer, and less ligand material is needed than if both the unwanted and the harmless ions were removed together.
Silica gel has been used as a silica substrate because silica gel has large surface areas. One kilogram of silica gel can have as much as 600,000 square meters of surface in the internal pores of the silica gel particle. It is estimated that one gram mole of an amine, when bound chemically in a single molecule thick layer, would cover approximately 350,000 square meters of silica gel surface. If the average complexation were one cation to each molecule of the amine, then the combination of a kilogram of silica gel bound to one gram mole of tetraethylenepentamine would complex and retain one gram mole of a heavy metal such as copper. The loading would be 63.5 grams of copper on one kilogram of dry silica gel. However, the pores of silica gel have small diameters, and it may be a slow process for molecules to penetrate them. Thus, it is possible that portions of the large surface area cannot be used to bind PEI and, consequently, will not be available to bind metal cations. In practice, the effective useable surface area of the silica gel is less than 350,000 square meters per kilogram, and cation complexing capacities of various tested silica gel immobilized simple amines, from the ethylenediamine to the tetraethylenepentamine are in the range of 0.05 to 0.3 gram moles of cation per kilogram of dry silica gel.
The use of coarse particles of silica gel, larger than those mentioned in the Ramsden patent, has been known to produce unsatisfactory results when bound to large amines such as PEI. It has been thought that coarse silica gel particles do not allow PEI molecules to diffuse adequately into them to produce high complexation capacities. In some cases, especially when the PEI has a molecular weight of about 50,000, the product is a gummy mass that has even lower complexation capacity, due to the adherence between particles. Some or all of these problems may account for the general belief in the art that PEI must be bound to relatively small silica gel particles.
It would be desirable to be able to effectively link PEI with larger particles of silica gel, especially those having a size ranging from about 200 mesh to about 20 mesh, which corresponds to a diameter range of about 74 microns to about 250 microns. Such larger particles would enable bound PEI to be employed in applications requiring less pressure drop than is created with smaller particles. Thus, for example, large volume applications would be possible.
Also, it would be desirable to employ a narrow screen fraction of relatively large silica gel particles with PEI. A narrow screen fraction eliminates excessive fines that might block interstices in a treatment bed.
Further, it would be desirable to have available a method of binding PEI to large particles of silica gel without producing a gummy mass. Generally, large volume applications are benefitted by maintaining a low pressure drop across the treatment bed.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, as embodied and broadly described herein, the class of compounds, method of manufacture, and method of use of this invention may comprise the following.